A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonance frequencies, with greater amplitude than at others. Resonators can be, for example, crystal oscillators (also known as quartz oscillators), inductance-capacitive (LC) oscillators, resistance-capacitive (RC) oscillators, and Microelectromechanical systems (MEMS) oscillators, also referred to as micromechanical MEMS oscillators. A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. Crystal oscillators, such as quartz oscillators, are commonly used to generate frequencies to keep track of time (as in quartz clocks) or to generate a clock signal for digital integrated circuits. Usually, a different crystal is required for each desired frequency. Also, the crystal and the oscillator circuit compliments are typically distinct from one another, i.e., they are not integrated.
For the past several years, MEMS structures have been playing an increasingly important role in consumer products. For example, it has been shown that radio-frequency (RF) surface micro-machined MEMS resonators were potential replacement part for quartz resonators in reference oscillator applications, as well as for other applications to keep track of time and to generate a stable clock signal for digital integrated circuits. The main advantage relies in the form factor and path to on-chip integration, but this advantage is balanced with the accuracy of the resonance frequency and higher temperature drift of MEMS resonators.
As these technologies mature, the demands on precision and functionality of the MEMS structures have escalated. For example, optimal performance may depend on the ability to fine-tune the characteristics of various components of these MEMS structures. Furthermore, consistency requirements for the performance of MEMS devices (both intra-device and device-two-device) often dictate that the processes used to fabricate such MEMS devices need to be extremely sophisticated.
In certain applications, the temperature stability and initial accuracy of resonators is particularly important, especially for MEMS resonators. Uncompensated MEMS resonators have a temperature coefficient that can be approximately forty parts per million per degrees Celsius (i.e., 40 ppm/° C.), whereas quartz oscillators can be approximately 0.035 ppm/° C. without any special compensation. For example, in the context of sleep clock applications, which use resonators with inherent accuracy of +/−20 ppm, quartz oscillators have tighter initial accuracy, smaller temperature drift, and can be fine tuned with capacitive pulling in the oscillator circuit, as compared to uncompensated MEMS oscillators. Whereas, state of the art quartz resonators, such as in wireless handset applications, exhibits frequency tolerance of +/−2 ppm after hand trimming involving deposition of infinitesimal quantity of metal at the quartz surface. For the same application, it has become clear that laterally vibrating bulk resonators were the structures of choice to fulfill application requirements in term of frequency, quality factor and power handling. But, in the case of surface micro-machined MEMS resonators, one can evaluate to the first order the fabrication tolerance on the absolute resonance frequency using tolerance on the lateral dimensions. State of the art lithography tools can achieve +/−15 nm in tolerance. For GSM/CDMA, typical resonator dimensions are in the range of 30-60 μm. This translates into an absolute tolerance of higher than 200 ppm. This already high value compared to quartz is assumed without taking into account effect of the stress in the package and other non-idealities in the micromachining process (anchors, alignment between layers, anisotropy, etc.) and makes a difficult trimming of the frequency necessary.
In terms of thermal characteristic, research has shown that surface micro-machined resonators (polysilicon, silicon-germanium, or piezoelectric) exhibit a temperature drift of more than 2500 ppm over a −30° C. to 85° C. range. This makes the use of compensation techniques developed for quartz crystal oscillators very difficult, where a typical AT-cut first mode resonator experiences a maximum excursion of +/−20 ppm over the full temperature range.
Some conventional approaches have been used in quartz oscillators to improve initial accuracy and temperature stability. One such conventional approach uses a varactor to pull a sleep clock resonant frequency to improve temperature stability, such as described in U.S. Pat. No. 6,160,458. However, this approach is not used in the context of MEMS oscillators. Other approaches may include electrostatic pulling in open-loop configuration, capacitive pulling through load capacitance, encapsulation of the device into micro-oven to keep temperature constant during fabrication, mechanical compensation of temperature drift, and Fractional-N PLL with reference drifting. Also, since uncompensated quartz oscillators have a lower temperature coefficient than MEMS oscillators, these conventional approaches are not used in a wide range of temperatures for temperature compensations of the oscillator.
Furthermore, traditional electrostatic pulling is not effective in high-frequency MEMS oscillators. High-frequency MEMS resonators, such as MEMS resonators having approximately 1 MHz frequency or greater, for example, have a very high equivalent stiffness that causes them to have a very small electrostatic frequency pulling range. In MEMS oscillators, capacitive pulling, like used in quartz-based oscillators, may also not be effective to adjust the output frequency for both initial accuracy and temperature stability due to extremely small effective capacitance of the MEMS resonators. For these reasons, new methods must be used to adjust the output frequency for both initial accuracy and temperature compensation over a wide range of temperatures for all types of resonators, such as quartz-based and MEMS oscillators.